Heavy lift lighter-than-air vehicles have been designed in recent years, owing largely to advances in high-strength fabric and composite materials, computerized flight control systems, propulsion systems and new fabrication techniques.
Throughout aviation history, the performance of very heavy lift by an air platform, i.e. the ability to lift a gross payload weight in excess of 250 tons via an air platform, long has been considered the exclusive domain of airships. The first significant heavy lift Airship design dates back to the 1.929 with the (U.S. Navy's ZMC-3, a metal clad Airship designed to carry as much as 100 tons. But this airship was never built. The ZMC-3 airship's proposed dimensions were 618 feet in length, and 154.5 feet in diameter. It was to carry 7,400,000 cubic feet of helium, with a gross lift of 212.5 tons, and a useful lift of 140.75 tons. Minus the weight of crew, spare parts and ballast, this airship was to have had a 100-ton capacity for cargo (more cargo lifting capacity than a present-day Air Force C-5A Galaxy). The ZMC-2's (its built predecessor) and known by sailors as the “Tin Bubble” advanced aeronautics technology, specifically the welding method to put together its aluminum envelope, that provided the American aeronautics industry with the industrial technology for fuselage assembly for U.S. warplanes during World War II.
On Jan. 31, 2006, Lockheed Martin's Skunk Works made the first flight of its “P-791” test bed at its facility on the Palmdale Air Force Plant 42 airport. The company did not announce or publicly discuss the flight. The P-791 is part of an independent research and development project to better understand airship capabilities and technologies, such as materials, Lockheed Martin officials say. The P-791 appears similar to the proposed full-scale version of the British SkyKitten, called the SkyCat. It may also be a quarter-scale prototype of this heavy-lifter, a hybrid airship envisioned with a 1000-ton payload capacity. Both airships have similar overall shapes, though the Skunk Works design is wider, and similar propulsion layouts, and both use air cushion landing gear. Perhaps the two programs have people in common. One of the partner names on the side of the P-791 is TCOM, an American manufacturer of aerostats and envelopes for airships. In the U.K., Mr. Roger Munk's Advanced Technologies Group (ATG) at Cardington, England built a 40-ft.-long unmanned SkyKitten (a smaller prototype of its Skycat) and flew it in 2000. The P-791 is a proof-of-principle vehicle to help engineers learn more about technology and aerodynamics for such airships.
But the P-791 is a unique platform that uses four air cushions located on its outer lobes as landing gear. Most of its lift comes from being filled with a lighter-than-air gas such as helium. But overall, it is heavier than air and gains the final 20% or so of lift by flying like an aircraft. The vehicle behaves like a flying hovercraft, except one with greater exposure to winds. During landing operations, the air cushions can be reversed to suck the aircraft onto the ground to resist winds for cargo operations. Air pressure also may be used to spread landing loads into the inflatable structure. Because the P-791 relies on air cushions it is completely ill suited for ship-to-ship at sea material transfer operations, since its huge downdrafts or updrafts would upset deck safety by generating venturi effects and FOD (foreign objects debris). Moreover, the P-791 lacks any external suspended loadframe with payload to safely land a ship's cargo deck or reach into a deep cargo holds. In sum, this airship with an internal cargo bay, is much too huge to land anywhere on a ship's deck.
In recent years, WALRUS, a heavier than air vehicle that utilizes heliLmi for buoyancy lift, has been designed to deliver a 500-1000 ton payload, was funded by DARPA for U.S. military applications. The WALRUS system is defined as a global airlift capability to deliver a brigade-size logistics payload over 12,000 nautical miles (nm) purportedly in fewer than 7 days from the continental U.S. (CONUS) “to unimproved landing sites and maritime environments”. In 2005, DARPA awarded Aeros Aeronautical Systems, a division of Worldwide Aeros, a 12-month $3,267,000 contract for phase I and awarded Lockheed Martin's Skunk Works a $2,989,779 contract for phase 1. WALRUS program manager Phil Hunt said: “This is not an airship. This is a heavier-than-air vehicle.”
The first critical challenge of WALRUS is the control of lift, which is to be generated in multiple ways. Much of the lift would be provided by lighter than air gas, such as helium, which could be superheated to increase buoyancy for take-off and supercooled for landing. “If we raise the temperature 35° C., we get an extra 15% lift,” Hunt said. Other ways of controlling buoyancy include ballonets inside the envelope, which can be filled with offboard air and then superheated or supercooled.
The second source of lift for WALRUS would be the aircraft's body and aerodynamic surfaces such as canards. “Techniques to change aerodynamic lift and reduce boundary-layer drag will be required”, said Hunt. The third source will be direct lift, either by vectoring the propulsion engines or by embedding thrusters in the airframe. “Each lift-producing mechanism has a different frequency of response, and they must be integrated to provide a “fly and forget” control system,” Hunt said.
More to the point, the WALRUS design is entirely ill suited for at sea cargo transfer operations, i.e. point to point transfer of very heavy payloads between ships underway or from ships to onshore facilities such as port or intermodal terminals and barges in a coastal or riverine environment. Having no suspended loadframe (with computerized rigging and stable platform to carry very heavy payloads over a target area for precise load on/load off) and being a heavier-than air aircraft measuring over 1,000 ft. in length, it would be far too long and dangerous to avoid ramming ship's superstructure. Being too big and bulky with no suspended loadframe, WALRUS is ill-suited for at-sea material transfer of very heavy cargoes.
Autonomous robotic airships were successfully designed and developed in the 1990s. The most important and integrated research and development related to unmanned, autonomous airships is the project AURORA, led by the Automation Institute of the Informatics Technology Center in Brazil. Other labs, particularly in France, have considered some particular problems related to airship autonomy, considering essentially the lowest level functionalities from a control theory point of view.
In the late 1990s, CargoLifter, a German cargo company, sought to fly off with a share of the estimated US $9 billion-a-year market in oversize cargo. CargoLifter spent millions of deutschmarks, mostly provided by the state of Brandenburg, in designing an advanced helium airship. But Cargolifter's CL75 and the larger version, the CL160, despite many technical innovations, both had a serious design flaw: they required water ballast. Active ballast control, therefore, was the greatest limiting factor to CargoLifter's airship operation.
CargoLifter designed two airships: the CL75 and CL160 (designed by the German company to carry 75 tons and 160 tons of cargo respectively). A prototype of the CL-75 was built: it was a single balloon with a diameter of 61 meters.
CargoLifter's CL160 was designed to haul up to 160 tons, around the weight of a Boeing 747, for a fraction of the cost of air freight and 10 times faster than the land and sea alternatives. CargoLifter was organized by Baron Carl von Gablenz. CargoLifter eventually went into receivership.
But the CL160 was never intended to perform point-to-point transfer of very heavy cargo at sea between ships underway. Today, Zeppelin has acquired the intellectual property of the now defunct CargoLifter organization. It will become part of a new Lighter-Than-Air Institute, headquartered in Friedrichshafen, for coordinating activities on scientific and predevelopment levels applicable to all types of airships.
Limiting the operational “footprint” of these ultra-large airships (ULAs) is a key issue. With the exception of the CL-160, that has a suspended external loadframe, all the mentioned ULAs have internal cargo bays to transport their cargo. This means that very heavy payloads must be loaded or offloaded from these heavy lift vehicles at super-size landing fields with level, stable surface of several hundred yards in length. Because they require a very large operational footprint, these ULAs are ill-suited for logistics operations in a compact area, like those required in Arctic Oil & Gas upstream operations or for cargo ships operating to and from a sea base. Indeed, none of the aforementioned heavy lift inflatables have been specifically designed to perform the in-air transfer of very heavy payloads between ships underway at sea.
The U.S. Navy currently transfers heavy payloads between ships underway by heavy lift helicopters such as the CH53E. But rotorcraft have limited payload lifting capacity. Typically, these “heavy lift” helicopters carry their loads on cables or “slings” extending beneath the fuselage of the aircraft. U.S. Marine Corp pilots who fly CH53E heavy lift helicopters complain that the suspended payload of 18 tons has a tendency “to fly itself” due to forward motion and downwash and other oscillations that produce a “springing effect”. Under adverse conditions of high wind or high sea states, suspended heavy payloads tend to become unmanageable due to cushioning effects and venturi reactions. The cruising speeds of such helicopters are also severely restricted by the externally slung, often low density, high drag loads. Dangerous load oscillations often develop as a result of load inertia and/or movements during flight. Such oscillations may result in load jettisoning or disastrous loss of flight control. Other hybrid rotorcraft, like the V-22 Osprey, a quad-tilt rotor design, while being able to vertically land and take off (VTOL) from flight decks, also has limited payload lifting capacity of about 20 tons.
The SkyCat operates as an independent platform. It can set down on sea if the winds and waves make it calm as glass. But it's too massive to set down on the deck of a moving ship and navigate around that ship's superstructure, in order to avoid collision, and its cargoes are stored inside the vehicle, rather than being suspended from above. In short it has no workable aircrane with suspended load mechanism for rapid payload transfer from ship to ship.
CargoLifter's CL160, intended to carry 160 tons of cargo may not be as radical a design as the SkyCat, but it was still a contender for the title of biggest aircraft ever constructed. At 260 meters, its design was 15 meters longer than the Hindenburg, which still holds the record. Like the SkyCat, the CL160 was to have a central keel and no internal frame. The keel, to be built from composites, would house a crew compartment, and, in the original design, support four 450-horsepower diesel engines and propellers, along with the cargo deck. Its designers saw the craft as a means to lift heavy and bulky items, such as generators and oil drilling equipment.
Both of these ultra large airships (ULAs) were tested by the U.S. Army in a wargame in 2001. The strategic ULA (SkyCat) immediately impacted the Vigilant Warriors 01 wargame with its ability to deliver a 750-short ton sustainment load, given the objective force's hand-to-mouth logistics capability. The requirement for at least a 3,000-toot open landing space, appropriate materials handling equipment, its size, and the fact that it is a civilian platform limited the ULA to certain locations. Floor restrictions on the aircraft limited cargo to lighter items such as helicopters, light vehicles, and sustainment stocks. It was, nonetheless, a valuable asset because of the amount of cargo it could deliver. The smaller, intratheater ULA (CL-75) could vertically deliver its cargo by hovering at approximately 100 meters and lowering its payload. The cost associated with the vertical discharge, however, was the requirement for a load exchange for ballast. In the wargame, ballast water was used, and this limited using CargoLifter's CL-75 to routes along the coast.
Active ballast control, therefore, was the greatest limiting factor to CargoLifter's airship operations.